The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit having layers containing an organic polymer called "kerogen," which upon heating decomposes to produce hydrocarbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the hydrocarbonaceous liquid product is called "shale oil."
The average kerogen content of formation containing oil shale can be determined by a standard "Fischer assay" in which a sample of core customarily weighing 100 grams and representing one foot of core drilled from the formation is subjected to controlled laboratory analysis involving grinding the sample into small particles which are placed in a steel vessel and subjected to heat at a known rate of temperature rise to measure the kerogen content of the core sample. Kerogen content is usually stated in units of "gallons per ton," referring to the number of gallons of shale oil recoverable from a ton of oil shale heated in the same manner as in the Fischer analysis. The average kerogen content of formation containing oil shale varies over a broad range from essentially barren shale having no kerogen content up to a kerogen content of about 70 gallons per ton. Localized regions can have even higher kerogen contents, but these are not common. It is often considered uneconomical to retort formation containing oil shale having an average kerogen content of less than about eight to ten gallons per ton.
Formation containing oil shale can be hundreds of feet thick. Often there are strata of substantial thickness within such formation having significantly different kerogen contents than other strata in the same formation. Thus, for example, in one formation containing oil shale in Colorado that is a few hundred feet thick, the average kerogen content is on the order of about 17 gallons per ton. Within this formation there are strata ten feet or so thick in which the kerogen content is in excess of 30 gallons per ton. In another portion of this same formation there is a stratum almost 30 feet thick having nearly zero kerogen content. Similar stratification of kerogen content occurs in many formations containing oil shale.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598 which are incorporated herein by this reference. Such patents describe in situ recovery of liquid and gaseous materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation and then fragmenting a portion of the remaining formation to form a stationary, fragmented permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen-containing retort inlet mixture into the retort as a gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the gaseous combustion zone feed into the combustion zone, the combustion zone is advanced through the retort.
The effluent gas from the combustion zone comprises combustion gas and any gaseous portion of the combustion zone feed that does not take part in the combustion process. This effluent gas passes through the fragmented mass in the retort on the advancing side of the combustion zone to heat oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and to a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by cooler particles in the fragmented mass on the advancing side of the retorting zone. The liquid products, together with water produced in or added to the retort, are collected at the bottom of the retort and withdrawn to the surface through an access tunnel, drift, or shaft. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, gas from carbonate decomposition, and any gaseous portion of the combustion zone feed that does not take part in the combustion zone process is also withdrawn to the surface.
Establishment of a combustion zone in the retort can be effected according to methods described in U.S. Pat. No. 4,027,917; U.S. Pat. No. 3,952,801; and U.S. patent application Ser. No. 810,491, filed on June 27, 1977, now U.S. Pat. No. 4,147,389, issued Apr. 3, 1979, which is assigned to the same assignee as the present application, all of which are incorporated herein by this reference.
U.S. Pat. No. 3,952,801 describes a technique for establishing a combustion zone in a retort by igniting the top of a fragmented permeable mass in the retort. According to this technique, a hole is bored to the top of the fragmented permeable mass and a burner is lowered through the bore hole to the oil shale to be ignited. A mixture of combustible fuel such as LPG (liquefied petroleum gas) and gas-containing oxygen, such as air, is burned in the burner and the resultant flame is directed downwardly toward the fragmented permeable mass. The burning is conducted until a substantial portion of the oil shale has been heated above its ignition temperature so combustion of oil shale in the fragmented mass is self-sustaining. Following ignition, introduction of fuel is terminated, the burner is withdrawn from the retort through the hole, and oxygen-supplying gas is introduced to the retort to advance the combustion zone through the retort.
U.S. Pat. No. 4,147,389 discloses a method for igniting and forming a combustion zone within the fragmented permeable mass of formation particles in an in situ oil shale retort. The method disclosed therein teaches forming a void above the upper boundary of the fragmented mass and placing in that void a combustible material such as particulate coal. The particulate combustible material placed in the void is more easily combustible than the fragmented permeable mass of formation particles containing oil shale. The combustible material is ignited by a burner lowered to the void for igniting such combustible material. Following ignition of the combustible material the operation of the burner is ceased. The heat generated from the combustion of the combustible material is utilized for forming a combustion zone within the fragmented mass of formation particles containing oil shale.
It can be time consuming to establish a combustion zone in a retort. For example, a start-up time as long as a week has been experienced with a retort in the south/southwest portion of the Piceance Creek structural basin in Colorado. Such a long start-up time results in consumption of large quantities of shale oil, LPG, or other processed fuel.
An in situ oil shale retort can have a substantial lateral extent. For example, it can be square with a lateral dimension of 160 feet or more. With such a large retort, a large number of burners and bore holes to various portions of the top of the retort and large quantities of fuel can be required for establishing a combustion zone in the retort. Preparation of a large number of bore holes and use of a large number of burners and large quantities of fuel can contribute significantly to the cost of producing liquid and gaseous products from oil shale.
It is desirable to maintain a combustion zone which is flat and uniformly transverse to the direction of advancement to maximize yield of products from the oil shale in an in situ oil shale retort. If the combustion zone is skewed relative to its direction of advancement, there is more tendency for oxygen present in the combustion zone to migrate into the retorting zone, thereby oxidizing products produced in the retorting zone and reducing the hydrocarbonaceous product yield in the liquid and gaseous products. In addition, excessive cracking of the hydrocarbonaceous products produced in the retorting zone can occur with a skewed and/or warped combustion zone. A combustion zone which is skewed and/or warped can be established if only a few burners are used for establishing the combustion zone. Use of more than a few burners to avoid a skewed or warped combustion zone can significantly increase the cost of establishing a combustion zone in a retort and producing shale oil.
Around each ignition point, or situs, in the fragmented permeable mass, a combustion zone is formed which tends to progress downwardly and laterally through the fragmented perameable mass. The combustion zone advances downwardly through the fragmented mass primarily resultant from convection of the hot gas flow through the retort and advances laterally and radially in the fragmented mass primarily by conduction and radiation. Since heat transfer by conduction and radiation through a fragmented mass of formation particles is much slower than heat transfer by convection, a substantial amount of unretorted oil shale can be left in the "corners" or side edges adjacent the walls of a retort. This can significantly reduce the yield of liquid and gaseous products obtained from the retort.
Thus it is desirable to provide a low cost and fast method for establishing a combustion zone in an in situ oil shale retort where the combustion zone is flat and uniformly transverse to its direction of advancement and extends laterally to the walls of the retort.